Pre-press color proofing is a procedure used by the printing industry for creating representative images of printed material without the high cost and time required to actually produce printing plates and set up a high-speed, high-volume, printing press to produce a single example of an intended image. These intended images may require several corrections and may need to be reproduced several times to satisfy customers requirements. By utilizing pre-press color proofing time and money can be saved.
One such commercially available image processing apparatus, disclosed in commonly assigned U.S. Pat. No. 5,268,708, describes image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media by applying a sufficient amount of thermal energy to the dye donor material to form an intended image. This image processing apparatus is comprised of a material supply assembly or carousel; lathe bed scanning subsystem, which includes a lathe bed scanning frame, translation drive, translation stage member, printhead, vacuum imaging drum, thermal print media and dye donor material exit transports.
The operation of the image processing apparatus comprises metering a length of the thermal print media, in roll form, from the material assembly or carousel. The thermal print media is cut into sheets, transported to the vacuum imaging drum, registered, wrapped around, and secured onto the vacuum imaging drum. A length of dye donor material, in roll form, is metered out of the material supply assembly or carousel, and cut into sheets. The dye donor material is transported to and wrapped around the vacuum imaging drum, such that it is superposed in the registration with the thermal print media.
After the dye donor material is secured to the periphery of the vacuum imaging drum, the scanning subsystem or write engine writes an image on the thermal print media as the thermal print media and the dye donor material on the spinning vacuum imaging drum is rotated past the printhead. The translation drive traverses the printhead and translation stage member axially along the vacuum imaging drum, in coordinated motion with the rotating vacuum imaging drum to produce the intended image on the thermal print media.
After the intended image has been written on the thermal print media, the dye donor material is removed from the vacuum imaging drum without disturbing the thermal print media that is beneath it. The dye donor material is transported out of the image processing apparatus by the dye donor material exit transport. Additional sheets of dye donor material are sequentially superposed with the thermal print media on the vacuum imaging drum, and imaged onto the thermal print media as described above until the intended image is completed. The completed image on the thermal print media is unloaded from the vacuum imaging drum and transported to an external holding tray on the image processing apparatus by the receiver sheet material exit transport.
The vacuum imaging drum is cylindrical in shape and includes a hollowed-out interior portion. A plurality of holes extending through the drums permit a vacuum to be applied from the interior of the vacuum imaging drum for supporting and maintaining the position of the thermal print media and dye donor material as the vacuum imaging drum rotates.
The outer surface of the vacuum imaging drum has an axially extending flat, which covers approximately eight degrees of the vacuum imaging drum circumference. The purpose of the axially extending flat is to assure that the leading and trailing ends of the dye donor material are partially protected from the effect of the air turbulence during the imaging process, since air turbulence has a tendency to lift the leading or trailing edges of the dye donor material. The vacuum imaging drum axially extending flat also ensures that the leading and trailing ends of the dye donor material are recessed from the vacuum imaging drum periphery. This reduces the chance that the dye donor material will contact other parts of the image apparatus, such as the printhead, which may cause a jam and loss of the intended image or worse, catastrophic damage to the image processing apparatus.
Although the presently known and utilized image processing apparatus is satisfactory, it is not without drawbacks. The donor and receiver media must be held tightly against the surface of the vacuum imaging drum as the drum rotates at high speeds. Near the surface of the rotating drum, a thin boundary layer condition exists in which the laminar flow of air effectively forms a very thin low-pressure region extending around the cylindrical surface of the drum. This boundary layer acts to provide a consistent low-pressure region on the outside of the film media, which is secured to the drum by vacuum. However, any irregularity in the drum surface, such as the axially extending flat, disturbs the laminar flow. This disturbance creates turbulence, in which the boundary layer separates from the drum surface. As a result, a region of high pressure is created, which can effectively slow drum rotation or lift an edge of the dye donor material, causing fly-off of the dye donor material and consequent damage to the image processing apparatus.
As the speed of drum rotation is increased, to increase production speed, this problem is exacerbated. One way to compensate for separation of the boundary layer, is to apply additional vacuum force to hold the leading and trailing edges of the film media against the drum more securely. Increasing the vacuum in the drum, however, requires increased drum thickness, a more heavy duty vacuum pump, and a more powerful drum motor, all of which adds expense.
Boundary layer control is an important consideration in design of aircraft. U.S. Pat. No. 4,664,345 (Lurz) for example, describes control of the boundary layer against an aircraft surface. Here, suction is employed to stabilize the boundary layer and prevent separation from a surface. U.S. Pat. No. 5,222,698 (Nelson et al.) also discloses use of suction to control boundary layer attachment and prevent turbulence. U.S. Pat. No. 5,535,967 (Beauchamp et al.) also describes using suction means to control a boundary layer and maintain laminar flow.
Boundary layer control along surfaces of rotating devices is not well known. U.S. Pat. No. 5,637,942 (Forni) discloses a method for boundary layer control in electric rotors and similar rotating devices. However, the method disclosed is for containing a boundary layer to effect drag reduction and control of axial air-flow for efficient motor operation.
The rotational speed of a vacuum imaging drum is one factor that determines overall throughput of an imaging apparatus. An improvement that allows higher drum speeds would help to increase throughput of the imaging apparatus. It can thus be seen that there is a need for maintaining the boundary layer and minimizing turbulence of surface air for an imaging apparatus that employs a vacuum imaging drum.